Process for the production of nodular cast iron



PROCESS FOR Tim PRODUQ'HBN F NODULAR CAST IRUN William H. Moore, New Rochelle, N.Y., assignor to Meehanite Metal Corporaiton No Drawing. Application October 3, 1958 Serial No. 765,051

2 Ciaims. (Cl. 75- 130) This invention relates to an improved method of making nodular cast iron and to the improved nodular cast iron made thereby.

In the past few years cast iron, with the graphite in the nodular and spheroidal form, has received a great deal of publicity and many patents have been granted dealing with various methods of producing nodular cast iron and with various alloys for producing nodular cast iron. it has been disclosed in US. Patent 2,527,037 that certain carbide metastabilizers of the nodular impelling type may be used to produce cast iron With nodular graphite, by a process where the gray cast iron is first whitened and is then graphitized.

It is also disclosed in British patent specification 53,573, that the nodular form of graphite may be produced by using telluriurn as a carbide metastabilizer in order to whiten the cast iron and then using magnesium silicon as a graphitizer.

This particular invention relates to an improved process utilizing the carbide stabilizing tendencies of tellurium and the graphitizing tendencies of magnesium silicon alloy. It is an object of this invention to provide an improved nodular cast iron. It is a further object of this invention to provide an improved method of controlling the formation of nodular graphite through the agency of tellurium and magnesium silicon. Other objects of this invention will be clear to those skilled in the art on reading the specification.

Tellurium has long been known for its ability to whiten a gray cast iron, even when only traces of telluriurn are present in the melt. Because of the extreme potency of tellurium in this direction, it has always been a matter of great difficulty to control the tellurium etfect and constantly produce either a fully pearlitic matrix, free from excess carbides, or nodular graphite controlled in precise degree and amount.

In the conventional methods of producing nodular cast iron, where magnesium is used in order to initiate the nodular form of graphite, tellurium has always been regarded as a subversive element. This is clearly outlined in US. Patent 2,485,760 where elements such as tellurium, selenium, arsenic, bismuth, antimony, lead and tin are regarded as subversive.

In the process of this invention tellurium is a necessary additive and its effects are beneficial rather than subversive.

The ability of telluriurn to produce surfusion, or undercooling, in a gray cast iron is, to a large extent, governed by the basic nature of this cast iron melt. Certain elements exercise an important influence on the effectiveness of tellurium as a whitener and this process depends on the discovery that manganese reduces and sulfur enhances the carbide metastabilizing tendencies of telluriurn. This fact hitherto has not been known to those skilled in the art.

It has been found that a high manganese content tends to limit the whitening ability of tellurium. This is somewhat surprising because manganese is in itself a whitener.

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The effect of manganese may be illustrated. by a simple test where a melt of cast iron was prepared having the following analysis:

Percent Total carbon 3.20 Silicon 2.23 Sulfur 0.075 Manganese 0.35 A portion of this melt was cast into a wedge test piece having a V2" base and a 28 /2" wedge angle. A wedge having a /2 base will be hereinafter referred to as a /2" wedge. The wedge value, as measured across the width of the line of demarcation between chill and gray, was found to be 3/ of an inch. A portion of thismelt was treated with tellurium in the amount of 0.01%. This so treated portion was also cast into a /2 wedge and the fracture of the wedge, in this case, indicated that it was all white.

A third portion of the same melt was treated with 0.01% of tellurium and this was followed with an addition of ferro-manganese in amount sufficient to give a final manganese content in the melt of 1.10%. This sotreated portion was cast into a /2" wedge and a measurement of the chill width at the line of demarcation showed the wedge value to be This test clearly indicates that manganese has a tendency to overcome the carbide stabilizing effect of tellurium.

It has been found that this effect of manganese in a tellurium-treated iron is somewhat modified by the presence of sulfur and oxygen in the melt. This is logical because sulfur and oxygen in the melt are usually in combination with the manganese content of the meltand would, therefore, determine the effectiveness of this manganese that is present in the melt.

In the production of nodular cast iron, where the flake graphite is first destroyed by the addition of tellurium to provide a carbidic iron and the melt is then graphitized with a magnesium-silicon, I have found that accurately predictable results can only be obtained where the'manganese and sulfur ratios are kept under adequate control.

The sulfur content of the melt seems to be particularly important in determining the stability of the chill imparted by the presence of tellurium in the melt. While the exact manner in which telluriurn, sulfur and manganese associate with each other in the melt is not known, it has been possible to work out a practical control procedure involving these elements.

i find that each 0.01% sulfur in the melt behaves like a 005% elluriurn addition to the melt. In the presence of a high sulfur content the stabiilty of the carbides produced by a tellurium addition is considerably greater. The easiest way to assess this sulfur eifect is in terms of the tellurium addition.

The effective tellurium addition is the actual telluriuin addition plus one half of the sulfur content of the melt. This is best illustrated by example.

The sulfur content of a melt was found to be 0.08%. To this melt an addition of 0.05% of telluriurn was made. The efiective tellurium addition (E.T.A.) would then be-- In another melt the sulfur content was 0.02% and the amount of telluriurn added to the melt was 0.06%. In this case the effective tellurium addition or E.T.A. was- 0.06% +(0.02+2) =0.06+0.0l or 0.07%

'The manganese content of the melt tends to decrease the stability of the carbides produced by tellurium and this may also be allowed for by a simple calculation.

I have found thateach 0.10% manganese in the range up to 1.0% manganese tends to graphitize about .005 "tellurium added. The manganese content of the iron (when this is under 1.00%) may be allowed for in terms of the equivalent tellurium addition or E.T.A. by subtracting .005% from the E.T.A. for each .10% manganese present. For example a melt having a sulfur content of 0.04% and a manganese content of 0.60% was treated with a tellurium addition of 0.07%. The equivalent tellurium addition (E.T.A.) would then be E.T.A.=0.07+ (0.04%-2) (0.005 X 6) If this same amount of tellurium were added to a melt having 0.12 sulfur and 0.50 manganese the equivalent tellurium addition would be E.T.A.=0.07+ (0.12-:-2)-(0.005 X) If, on the other hand, the sulfur content of this melt were 0.12% and the manganese content was 1.0% then the equivalent tellurium addition would be- E.T.A.=0.07+(0.12+2)(0.005 X 10) The first step in the process of this invention is to calculate the equivalent tellurium addition so that the effect of the manganese and sulfur contents of the melt, in terms of carbide metastability, may be allowed for.

In the process of this invention a predetermined amount of tellurium is'added to the melt but the elfect of this addition is then calculated in terms of the E.T.A. described.

I find that the tellurium addition made to the melt should be sufiicient to produce a degree of whiteness corresponding to 100-150% of the casting section to be poured. Thus, in terms of test wedges for example a casting of 1 inch or would be required to have an initial wedge value, produced by the tellurium addition, of from 32-48/ 32 to ensure this degree of whiteness.

The amount of whiteness suitable for most purposes may be obtained with a .06% addition of tellurium. This amount is then properly adjusted for efiective tellurium addition according to the method already outlined. For heavier casting sections I prefer to increase this amount slightly and for lighter sections I prefer to reduce this slightly.

Thus for casting sections below about one inch I prefer to use a tellurium addition of about 0.05%, for castings from one inch to about 3 inches a tellurium addition of about 0.06% and for castings above 3 inches a tellurium addition of about 0.07%. In every case this actual addition is converted to an efiective addition, according to the procedure already outlined. The actual addition should be sufficient to causethe degree of whitening, whereasthe effective addition is used for gauging the second step of the process, which is that of graphitizing, by the addition of a magnesium silicon alloy.

The second step of this invention involves the addition of a magnesium silicon alloy as a graphitizer.

Because of the volatility of magnesium and the pyrotechnics that accompany an addition of magnesium alloy to the melt, I prefer to use an alloy containing about 10% magnesium with the silicon content being at least 30% and the. balance usually iron. Other alloy combinations of magnesium and silicon may be used and in these cases the amounts necessary for adequate graphitization would be difierent. For the examples quotedin this specification the alloy containing about 10% magnesium was used.

I find that the proper amount of graphitizer in most cases amounts to preferably 50 times the amount of elfective tellurium that has been added to the melt however acceptable results have been obtained with amounts in the range of from 30 to 70 times the amount of effective tellurium that has been added to the melt. Thus where the effective tellurium added has been 0.06% the amount of magnesium silicon normally required for graphitization would be about 50 0.06% or 3.0% by weight. Where the E.T.A. has been 0.08% the amount of magnesium silicon required would be 50 '0.08% or 4.0%.

Where insuflicient magnesium silicon is added graphitization is incomplete and free carbides may remain in the melt. This would usually only be desirable for special purposes such as where the casting is to be used for an application requiring hardness. Where too much magnesium silicon is added the mechanical properties may be impaired and costs would be increased unnecessarily.

In most cases where carbide metastability is involved the original composition of the bath in terms of carbon equivalent or natural graphitizing power is an important factor. Usually a high carbon equivalent would require a higher addition of carbide metastabilizer agent to ensure whiteness and perhaps a lower graphitizing addition to ensure grayness. This is true only to a limited extent in the process of this invention. I find that tellurium is so effective in promoting carbide stability that it is not necessary to adjust the tellurium addition to suit the carbon equivalent of the melt.

The only adjustment necessary is to add a little extra magnesium silicon for the purposes of graphitizing where the carbon equivalent of the melt is low, i.e. where the bath is strongly hypoeutectic. This addition would seldom amount to more than 10% of the amount usually added. By the same token any adjustment for a strongly hypereue tectic bath would seldom amount to less than 10% of the amount usually added.

The actual working of this invention may be demonstrated by actual example. In a first example a bath was melted having the following compositionthe molten bath. This addition corresponds to an equivalent tellurium. addition (E.T.A.) of- 0.06+ (0.06:-2) (0.005 X5) or 0.06+0.03-0.025 or 0.065%

A 2" wedge, that is a wedge having a base of 2", was cast from this tellurium treated melt and was found to be white to the extent of The melt was then graphitized with magnesium silicon (10% magnesium) in the amount of 0.065 X50 or 3.25% by weight. A test wedge cast from this graphitized melt exhibited a carbide value of The melt was cast and a test piece was taken from the casting. The test piece contained all the graphite in the nodular form and exhibited a tensile strength of 78,250 p.s.i. with an elongation of 5.5% in the as cast condition.

In a second example a bath was melted to the following This melt was to be poured into a casting having a section so the melt was treated with a tellurium addition of 0.05%. This corresponds to an equivalent tellurium addition of 0.05 (0.02-:2) (0.005 X7) or 0.05 +0.01 0.035 or 0.025

The melt was then graphitized with magnesium silicon in amount equal to 50 0.025 or 1.25%. The resultant casting was found to contain all the graphite in nodular form.

The product of this invention has several inherent advantages not normally exhibited by nodular irons. It can be cast fully pearlitic in even relatively heavy casting sections, it is singularly free from dross inclusions and it has predictable physical properties. Where the carbide metastability is not adjusted to suit the chemistry, particularly with regard to sulfur and manganese, the production of nodular iron by the process of whitening with tellurium and graphitizing with magnesium silicon is a matter of chance.

This invention has been described with a certain degree of particularity in its preferred form, but it should be understood that the present disclosure of the preferred form has been made only by way of example and that numerous change; in the details of construction and operation may be made without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An improved method of producing nodular cast iron from a gray cast iron melt comprising the steps of adding to the melt an amount of tellurium sufiicient to produce a white melt and then graphitizing: the melt with magnesium silicon alloy in an amount equal to a value lying in the range of from to times the equivalent tellurium addition, said equivalent tellurium addition being the actual tellurium addition, plus one-half of the sulphur content of the melt minus 0.05 times the manganese content of the melt, and finally casting said treated melt.

2. An improved method of producing nodular cast iron from a gray cast iron melt comprising the steps of adding to the melt an amount of tellurium equal to .05 for casting sections of 1" and under, .06% for casting sections of between 1" and including 3", and .07 for casting sections greater than 3", then adding to the melt magnesium silicon alloy in amount equal to a value lying in the range of from 30 to 70 times the equivalent tellurium addition, said equivalent tellurium addition being the actual tellurium addition, plus one-half of the sulphur content of the melt minus 0.05 times the manganese content of the melt, and finally casting said treated melt.

References Cited in the file of this patent FOREIGN PATENTS 680,123 Great Britain Oct. 1, 1952 

1. AN IMPROVED METHOD OF PRODUCING NODULAR CAST IRON FROM A GRAY CAST IRON MELT COMPRISING THE STEPS OF ADDING TO THE MELT AND AMOUNT OF TELLURIUM SUFFICIENT TO PRODUCE A WHITE AND MELT AND THEN GRAPHITIZING THE MELT WITH MAGNESIUM SILICON ALLOY IN AN AMOUNT EQUAL TO A VALUE LAYING IN THE RANGE OF FROM 30 TO 70 TIMES THE EQUIVALENT TELLURIUM ADDITION, SAID EQUIVALENT TELLURIUM ADDITION BEING THE ACTUAL TELLURIUM ADDITION, PLUS ONE-HALF OF THE SULPHUR CONTENT OF THE MELT MINUS 0.05 TIMES THE MANGANESE CONTENT OF THE MELT, AND FINALLY CASTING SAID TREATED MELT. 